Method and apparatus providing common path distortion (CPD) detection from a field instrument

ABSTRACT

A method and an apparatus providing common path distortion (CPD) detection from a field instrument, particularly when the source of the common path distortion is at a portion of the network beyond the subscriber&#39;s tap.

BACKGROUND

In a cable system, a network of interconnected electrical cables,referred to as a cable plant, is commonly used to deliver information tosubscribers. Most cable network systems are coaxial-based broadbandaccess systems that may take the form of all-coax network systems,hybrid fiber coax (HFC) network systems, or RF over glass (RFOG) networksystems. Cable network system designs, including, for example, cabletelevision (CATV) network system designs, typically use atree-and-branch architecture that permits bi-directional datatransmission, including Internet Protocol (IP) traffic between the cablesystem head-end and customer locations. There is a forward or downstreamsignal path (from the cable system head-end to the customer location)and a return or upstream signal path (from the customer location back tothe cable system head-end). The upstream and the downstream signalsoccupy separate frequency bands. The frequency range of the upstreamband is from 5 MHz to 42 MHz, 5 MHz to 65 MHz, 5 MHz to 85 MHz, or 5 MHzto 204 MHz, while the downstream frequency band is positioned in a rangeabove the upstream frequency band.

Customer locations may include, for example, cable network subscriber'spremises. Typical signals coming from a subscriber's premises include,for example, set top box DVR/On Demand requests, test equipment datachannels, and Internet Protocol output cable modem carriers defined bythe Data Over Cable Service Interface Specification (“DOCSIS”), which isone communication standard for bidirectional data transport over a cablenetwork system.

FIG. 1 illustrates an example cable plant 10, which may be part of amuch larger network such as e.g., a cable TV network that delivers cableTV signals, including digital TV signals and data and control signals,to end users at customer premises 22, 26 in the downstream direction. Inaddition, the cable plant 10 may receive and forward data and othersignals from the customer premises 22, 26 in the upstream direction.

The cable plant 10 may include one or more trunk cables 12, which may beburied in the ground or elevated above the ground on e.g., utilitypoles, or a combination of both. The one or more trunk cables 12 mayoriginate from a headend of the cable network (not shown). The headendmay include a cable modem termination system (CMTS), which handlessignals on the one or more trunk cables 12 and performs functions knownto be performed by headends in cable TV networks. Subscriber tap ports14, 16 and drops 18, 20 may be used to connect one of the trunk cables12 to the customer premises 22, 26, respectively. At the customerpremises 22, 26, the downstream signals may be demodulated using cablemodems or set top boxes (not shown), which may be connected to othercustomer premises equipment (not shown), such as wireless routers, smarttelevisions, personal computers, smartphones, etc.

Increased bandwidth utilization and the bidirectional use of cableplants 10 have increased the sensitivity of cable networks to networkimpairments. One such impairment, affecting mostly upstream signals, iscommon path distortion (CPD). Common path distortion is a problem incable networks where signals pass through a portion of the network thatdistorts the signals in a non-linear way. When the signals pass throughthis non-linear effect, they mix and produce new signals that are thesecond and third order intermodulation products. When these signals fallwithin the return path, they can travel back to the headend. Due to thefunnel effect of the upstream path, a small noise contribution from manydistinct sources will combine together and can create a significantlyelevated noise floor at the headend.

Although common path distortion varies in severity and manifests itselfin many different ways, it has a very distinctive spectral signature.Typically, common path distortion is characterized by a significant riseof the noise floor across the upstream spectral band. The rise of thenoise floor is accompanied by spectral beats spaced apart at e.g., 6 MHzintervals. The spectral beats also occur in the upstream spectral band.Common path distortion can cause a major reduction ofcarrier-to-impairment power ratios, leading to errors in upstreamdigital transmissions, which is undesirable.

As shown in FIG. 2, common path distortion can be generated in theportion of the network downstream from the customer tap port 14 as theresult of e.g., a corroded ground block 24. In the illustrated example,downstream signals can cause common path distortion when they mix in thecorroded ground block 24. A portion of the common path distortion is inthe upstream signals, potentially affecting all premises 22, 26connected to the trunk and or network nodes.

Finding the sources of common path distortion can be challenging.Especially, as they move closer to the network edge. As shown in FIG. 3,traditional upstream spectrum measurements using a conventional testinstrument 50 do not work when the CPD source is located on the customerside. This is because the cable drop 18 needs to be disconnected fromthe tap port 14 (and hence the network) and connected to the testinstrument 50. Since the return path noise is caused by the forward pathsignals (i.e., downstream signals), the noise stops once the cable isdisconnected—even though the underlying defect (e.g., corroded groundblock 24) is still present.

As shown by the example ingress scan level versus frequency graph 40,the test instrument 50 would not detect a failure since the noise (asshown by line 42) stops and or is below a predetermined pass/failthreshold level (as shown by line 44). This false test result isundesirable. It is also a waste of time and labor costs associated witha servicing technician who is trying to detect the source of the CPD.Significantly, once the drop 18 is reconnected to the tap port 14 andthe network, the noise source and CPD will return because the forwardpath signals are once again present. This situation is also undesirable.

It may be possible to detect common path distortion if a reverse testport and an additional drop is added to the cable plant 10 (e.g.,between the customer's premises 22 and the one or more trunk cables 12)and hardware changes are made to the test instrument 50. As can beappreciated, this would increase the cost of the cable plant 10 andnetwork, particularly if this additional equipment was added for eachcustomer in the network. Thus, this potential solution is alsoundesirable.

Accordingly, there is a need and desire to detect common path distortionusing a test instrument, particularly when the source of the common pathdistortion is on the downstream side of the network (i.e., the portionof the network beyond the subscriber's tap).

SUMMARY

Embodiments described herein may be configured to provide common pathdistortion detection from a field instrument and when the source of thecommon path distortion is on the downstream side of the network. As usedherein, “downstream side of the network” is the portion of the networkbeyond the subscriber's tap. In one embodiment, a computer-implementedmethod is provided. The method is performed on a test instrument adaptedto test for common path distortion at a downstream portion of a cabletelevision network and may comprise: transmitting a first test signalhaving a first frequency into the downstream portion of the network;transmitting a second test signal having a second frequency into thedownstream portion of the network; measuring a first input signal at athird frequency; measuring a second input signal at a fourth frequency,the third and fourth frequencies being different frequencies and basedon the first and second frequencies; and determining that the downstreamportion of the network is experiencing common path distortion based on acharacteristic of the first or second input signals.

In another embodiment, a test instrument for testing for common pathdistortion at a downstream portion of a cable television network isprovided. The test instrument comprises a storage device; and aprocessor executing program instructions stored in the storage deviceand being configured to determine transmit a first test signal having afirst frequency into the downstream portion of the network; transmit asecond test signal having a second frequency into the downstream portionof the network; measure a first input signal at a third frequency;measure a second input signal at a fourth frequency, the third andfourth frequencies being different frequencies and based on the firstand second frequencies; and determine that the downstream portion of thenetwork is experiencing common path distortion based on a characteristicof the first or second input signals.

In one or more embodiments, the third frequency is a combination of thefirst and second frequencies and the fourth frequency is the differencebetween the first and second frequencies. In one or more embodiments, aquiet zone is determined and the first and second frequencies areselected such that the third and fourth frequencies fall within thequiet zone.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an example of a conventional cable plant.

FIG. 2 shows the example cable plant of FIG. 1 experiencing common pathdistortion in a portion of the network downstream from a customer tapport.

FIG. 3 shows the example cable plant of FIG. 1 undergoing a theoreticalcommon path distortion trouble shooting method using a conventional testinstrument.

FIG. 4 shows the example cable plant of FIG. 1 undergoing one of thedisclosed common path distortion detection methods using a testinstrument constructed in accordance with the disclosed principles.

FIG. 5 shows a first example common path distortion detection method inaccordance with the disclosed principles.

FIG. 6 shows the example cable plant of FIG. 1 undergoing the examplecommon path distortion detection method illustrated in FIG. 5.

FIG. 7 shows a second example common path distortion detection method inaccordance with the disclosed principles.

FIGS. 8 and 9 show examples for determining where the test instrumentmay or may not intentionally select “quiet zones” in accordance with thedisclosed principles.

FIG. 10 shows a graph illustrating one example method for determiningbaseline noise that may be used in a common path distortion detectionmethod in accordance with the disclosed principles.

FIG. 11 shows an example of a test instrument for performing the commonpath distortion detection methods of FIGS. 5 and 7 in accordance withthe principles disclosed herein.

DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS

The disclosed principles provide a technique to detect common pathdistortion using a test instrument that may be particularly well suitedfor situations when the source of the common path distortion is on thedownstream side of a network. In one or more embodiments, the disclosedmethod and test instrument couples together the instrument's measurementcapability with its transmit capability to perform a unique CPDdetection method. For example, when the customer side cable drop isconnected to the test instrument and disconnected from the cable plantand network, predetermined signals can be output from the instrument'stransmitter and into the premises via the drop. The test instrument maythen take measurements at one or more frequencies where signals areexpected to be present if there was a non-linear effect (e.g., CPD) inthe network. If the test instrument detects the expected signals, thenthe source of the common path distortion is near and the technician mayperform additional testing at different locations at the premises tolocate the source.

FIG. 4 shows the example cable plant 10 of FIG. 1 undergoing one of thedisclosed common path distortion detection methods (e.g., method 200 ofFIG. 5, method 250 of FIG. 7) using a test instrument 100 constructed inaccordance with the disclosed principles. In one embodiment, the testinstrument 100 is one of the OneExpert CATV line of analysis metersmanufactured and sold by VIAVI Solutions Inc. that is modified toperform the processing disclosed herein. In one or more embodiments, themodifications can be made by a software/firmware upgrade.

As shown in FIG. 4, the cable drop 18 to the customer premises 22 isdisconnected from the tap port 14 (and hence the network) and connectedto the test instrument 100. In this configuration, and in accordancewith the disclosed principles, the test instrument 100 will be able togenerate, transmit, measure and process signals to help locate thesource of common path distortion even though the source (e.g., corrodedground block 24) is located at the downstream side of the network. Thisis a substantial improvement over the conventional test instrument 50(FIG. 3), which could not locate the source of common path distortion inthis situation (i.e., when the customer drop 18 is disconnected from thetap port 14 and the source is on the downstream side of the network).

FIG. 5 shows a first example common path distortion detection method 200in accordance with the disclosed principles and FIG. 6 shows the cableplant 10 of FIG. 1 undergoing method 200 using a test instrument 100programmed to perform the method 200. In one embodiment, the method 200may be activated by a technician and performed as part of a “CPD Check”performed by the test instrument 100. The method 200 may be activatedusing one or more buttons or a touchscreen included with the instrument100 (described below in more detail with respect to FIG. 11). Regardlessof how it is activated, the method 200 should be executed after thecable drop 18 to the customer premises 22 is disconnected from the tapport 14 (and hence the network) and connected to the test instrument100.

At step 202, and as shown in the amplitude v. frequency graph 110 ofFIG. 6, the test instrument 100 may transmit a first test signal havinga first predetermined frequency F1 (as shown by line 112) and a secondtest signal having a second predetermined frequency F2 (as shown by line114) to the drop 18 and hence the customer's premises 22. For example,two continuous wave signals may be output with the first having afrequency F1 equal to 9 MHz and the second having a frequency F2 equalto 16 MHz.

As discussed above, the test instrument 100 may be programmed to takemeasurements at one or more frequencies where signals are expected to bepresent if there was a non-linear effect (e.g., CPD) in the network. Inthe illustrated example, at step 204, the test instrument 100 isprogrammed to take a first input signal measurement at a frequency M1equal to F2−F1 (as shown by line 116) and a second input signalmeasurement at a frequency M2 equal to F2+F1 (as shown by line 118). Inkeeping with the current example, where the first test signal is outputat a first frequency F1 of 9 MHz and the second test signal is output ata second frequency F2 of 16 MHz, the test instrument 100 may make thefirst measurement at frequency M1 equal to F2−F1 (i.e., 16 MHz-9 MHz) or7 MHz and the second measurement at frequency M2 equal to F2+F1 (i.e.,16 MHz+9 MHz) or 25 MHz to see if there are signals present from themixing of the two test signals and their frequencies.

In one or more embodiments, a signal is present at a measurementfrequency M1, M2 if the signal has an amplitude above a predeterminedthreshold. In accordance with the disclosed principles, if the testinstrument 100 measures signals having an amplitude above apredetermined threshold at the M1, M2 frequencies, it has detected theexpected signals, meaning that the source of the common path distortionis near. In one or more embodiments, the predetermined threshold couldbe set to 0 (i.e., any signal present at the M1, M2 frequencies isevidence of CPD). In one or more embodiments, the predeterminedthreshold could be set to a noise floor or a baseline noise level (i.e.,any signal present at the M1, M2 frequencies above the noise floor orbaseline noise level is evidence of CPD).

In one embodiment, at step 206, the test instrument 100 may store and oroutput the measured signals and or their amplitudes for evaluation bythe technician. In one or more embodiments, the test instrument 100could output an affirmative indicator to alert the technician thatmeasured signals at frequencies M1, M2 are present and the source of theCPD may be near. For example, the test instrument 100 could provide avisual indication of the amplitude of the measured signals atfrequencies M1, M2 (e.g., as shown in graph 110). In addition to, oralternatively, the test instrument 100 could provide a message orgraphical indicator to alert the technician that the measured signals atfrequencies M1, M2 exceed the threshold. Moreover, or alternatively, thetest instrument 100 could output audible and or haptic (e.g., vibration)indicators when the measured signals at frequencies M1, M2 exceed thethreshold.

Regardless of the output at step 206, the technician may repeat method200 after connecting the test instrument 100 to a different connectionpoint within the customer's premises 22. This will allow the technicianto perform additional testing at different locations at the premises 22to hone in on and or locate the source of the CPD. In one or moreembodiments, the test instrument 100 could perform the method 200 usingmultiple different frequencies to determine if there is any frequencyselectivity to the non-linear effect. That is, the test instrument 100could be programmed to transmit signals having frequencies F1, F2 otherthan 9 MHz and 16 MHz, meaning that it would be programmed to measureexpected signals at different M1, M2 frequencies.

FIG. 7 shows a second example common path distortion detection method250 in accordance with the disclosed principles. The test configurationwould be similar to the configuration shown in FIG. 4 except that thetest instrument 100 is programmed to perform method 250. In oneembodiment, the method 250 may be activated by a technician andperformed as part of a “CPD Check” performed by the test instrument 100.The method 250 may be activated using one or more buttons or atouchscreen included with the instrument 100 (described below in moredetail with respect to FIG. 11). Regardless of how it is activated, themethod 250 should be executed after the cable drop 18 to the customerpremises 22 is disconnected from the tap port 14 (and hence the network)and connected to the test instrument 100.

In this example method 250, the test instrument's 100 ingressmeasurement capability will be utilized to determine the frequenciesthat the test signals should be transmitted at (i.e., F1, F2) andmeasured at (i.e., M1, M2). In one or more embodiments, a standardingress measurement may be taken and used to identify one or morefrequencies within a measurement pass band having the smallest amount ofingress noise. One or more of these frequencies could be chosen and usedfor the frequencies F1, F2 of the test signals output by the transmitterso that any resulting expected measured signal would be placed withinthese “quiet” zones. As can be appreciated, this may provide additionaldepth of the measurements and certainty that the measured signals arefrom the intermixing of the two test signals and not some other sourceof ingress.

To this end, at step 252, the test instrument may measure ingress at thecustomer's premises 22. Any type of ingress measurement may beperformed. For example, if the test instrument 100 is a OneExpert metermodified to perform the processing disclosed herein, step 252 may beperformed using its OneCheck functionality. At step 254, the testinstrument 100 may determine if the measured ingress exceeds apredetermined ingress threshold or not. If it is determined that themeasured ingress does not exceed the predetermined ingress threshold(i.e., a NO at step 254), the method 250 continues at step 256 where thefrequencies F1, F2 of the test signals and frequency of the measuredsignals M1, M2 are set to default values. That is, because the ingressscan of step 252 is good (i.e., low ingress from the premises, flatacross frequency, etc.), the method's 250 measurements at frequenciesM1, M2 would not be affected by ingress noise and the test signalfrequencies F1, F2 most likely do not need to be adjusted from thedefault values. This situation is illustrated in example 400 shown inFIG. 8. As can be seen, the example 400 shows amplitudes of the ingress(shown by line 402) as being substantially flat across the frequencyrange of 5 MHz to 110 MHz. In one or more embodiments, the frequenciesF1, F2 selected at step 254 may be the same frequencies discussed abovewith respect to method 200.

If, however, it is determined that the measured ingress exceeds thepredetermined ingress threshold (i.e., a YES at step 254), the method250 continues at step 258 where the frequencies F1, F2 of the testsignals and frequencies M1, M2 of the measured signals are selected inan attempt to avoid ingress noise from affecting the subsequentmeasurements at the measured frequencies M1, M2. That is, because theingress scan shows that there is ingress coming from the premises 22, anattempt is made to find frequencies where the ingress is the lowest (orexclude frequencies where the ingress is high/higher than at otherfrequencies) to find regions (referred to herein as “quiet zones”) tomake the measurements with little or no impact from ingress noise.

For example, as shown by example 450 illustrated in FIG. 9, there arethree frequency ranges 452, 456, 460 where the ingress is flat. Inaddition, these frequency ranges 452, 456, 460 are relatively low incomparison to frequency ranges 454, 458, 462 displaying elevated andspiked ingress noise (i.e., “noisy regions”). In one embodiment, thefrequencies F1, F2 of the test signals may be chosen such that thefrequencies of the expected measured signals M1, M2 fall within one ofthe quiet regions (i.e., frequency ranges 452, 456, 460). In addition,or alternatively, the frequencies F1, F2 of the test signals may bechosen such that the frequencies of the expected measured signals M1, M2do not fall within one of the noisy regions (i.e., frequency ranges 454,454, 462).

Once the frequencies F1, F2 of the first and second test signals areselected (whether at step 256 or 258), the method continues at step 260where the test instrument 100 transmits the first test signal atfrequency F1 and the second test signal at frequency F2. The testinstrument 100 may be programmed to make measurements at one or morefrequencies M1, M2 where signals are expected to be present if there wasa non-linear effect (e.g., CPD) in the network. In the illustratedexample, at step 262, the test instrument 100 is programmed to take afirst input signal measurement at a frequency M1 equal to F2−F1 and asecond input signal measurement at a frequency M2 equal to F2+F1.

In one or more embodiments, a signal is present at a measurementfrequency M1, M2 if the signal has an amplitude above a predeterminedthreshold. In accordance with the disclosed principles, if the testinstrument 100 measures signals having an amplitude above apredetermined threshold at the M1, M2 frequencies, it has detected theexpected signals, meaning that the source of the common path distortionis near. In one or more embodiments, the predetermined threshold couldbe set to 0 (i.e., any signal present at the M1, M2 frequencies isevidence of CPD). In one or more embodiments, particularly when ingressis detected at step 254, the predetermined threshold could be set to anoise floor or a baseline noise level (i.e., any signal present at theM1, M2 frequencies above the noise floor or baseline noise level isevidence of CPD). FIG. 10 shows a graph 500 illustrating one examplemethod for determining baseline noise that may be used during athreshold determination in step 262 in accordance with the disclosedprinciples. In accordance with the disclosed principles, the ingressscan (step 252) can be used to establish baseline noise, which can besubtracted from the measured values. The relative difference between themeasured signal values and the baseline may be used for thresholdingpurposes. The example graph 500 includes one line 502 representingmeasured signal amplitude over a frequency range and a second line 504representing measured ingress amplitude over the frequency range. Point506 is the most relevant portion of the example graph 500 because itshows the biggest difference between the measured signals and ingresslevel.

In one embodiment, at step 264, the test instrument 100 may store and oroutput the measured signals and or their amplitudes for evaluation bythe technician. In one or more embodiments, the test instrument 100could output an affirmative indicator to alert the technician thatmeasured signals at frequencies M1, M2 are present and the source of theCPD may be near. For example, the test instrument 100 could provide avisual indication of the amplitude of the measured signals atfrequencies M1, M2. In addition to, or alternatively, the testinstrument 100 could provide a message or graphical indicator to alertthe technician that the measured signals at frequencies M1, M2 exceedthe threshold. Moreover, or alternatively, the test instrument 100 couldoutput audible and or haptic (e.g., vibration) indicators when themeasured signals at frequencies M1, M2 exceed the threshold.

Regardless of the output at step 264, the technician may repeat method250 after connecting the test instrument 100 to a different connectionpoint within the customer's premises 22. This will allow the technicianto perform additional testing at different locations at the premises 22to hone in on and or locate the source of the CPD.

FIG. 11 shows a high-level block diagram of the test instrument 100,according to an example embodiment. It should be appreciated that thetest instrument 100 may include components other than those shown. Thetest instrument 100 may include one or more ports 303 for connecting thetest instrument 100 to a tap, such as the tap ports 14, 16 shown inFIGS. 1-4 and 6. The one or more ports 303 may include connectors forconnecting to cables in the cable plant 10 and the network that carrytraffic for upstream and downstream channels. The traffic may includevideo, voice and data packets, etc. The test instrument 100 may includea telemetry interface 304 for connecting to a telemetry channel, such asa WiFi interface, Bluetooth interface, cellular interface or anothernetwork interface. The test instrument 100 may connect to a remotedevice via the telemetry interface 304.

The test instrument 100 may include a user interface, which may includea keypad 305 and display 313. The display 313 may include a touch screendisplay. A user may interact with the test instrument 100, such as toenter information, select operations, view measurements, viewinterference profiles, etc., via the user interface.

A data storage 351 may store any information used by the test instrument100 and may include memory or another type of known data storage device.The data storage 351 may store measured signal data, ingress signaldata, noise floor data, thresholds and/or any other measurements or dataused by the test instrument 100, particularly the data required formethods 200 and 250. The data storage 351 may include a non-transitorycomputer readable medium storing machine-readable instructionsexecutable by processing circuit 350 to perform operations of the testinstrument 100 such as those described for method 200 and method 250.

Transmission circuit 341 may include a circuit for sending test signalsupstream to perform various tests, such as frequency sweep tests. Thetransmission circuit 341 may include encoders, modulators, and otherknown component for transmitting signals over the cable plant 10 andwithin the network. Receiver circuit 342 may include components forreceiving signals from the cable plant 10 and network. The componentsmay include components such as a demodulator, decoder, analog-to-digitalconverters, and other known components suitable for a receiver circuit.

Processing circuit 350 may include any suitable hardware to perform theoperations of the test instrument 100 described herein, including theoperations described with respect to FIGS. 5 and 7 and the methods 200,250 described herein. The operations may include transmitting testsignals at desired frequencies measuring and testing operations andcalculations described herein. The hardware of the test instrument 100,including the processing circuit 350, may include a hardware processor,microcontroller, a digital signal processor (DSP), an applicationspecific integrated circuit (ASIC), a field programmable gate array(FPGA) or other programmable logic device, discrete gate or transistorlogic, discrete hardware components, or any combination thereof designedto perform the functions and methods described herein. In an example,one or more of the functions and operations of the test instrument 100described herein may be performed by the processing circuit 350 or otherhardware executing machine readable instructions stored in anon-transitory computer readable medium, which may comprise RAM (randomaccess memory), ROM (read only memory), EPROM (erasable, programmableROM), EEPROM (electrically erasable, programmable ROM), hard drives,flash memory, or other types of storage devices, which may be volatileand/or nonvolatile.

The apparatus for implementing a common path distortion detection methoddisclosed herein provides numerous advantages over the current state ofthe art. For example, the test instrument and methods disclosed hereincan help locate the source of common path distortion even when thesource is located at the downstream side of the network. This is asubstantial improvement over conventional test instruments, which couldnot locate the source of common path distortion in this situation. As aresult, testing is more accurate than the conventional testing andbetter reflects the true performance of the network and its source oferrors. This reduces the possibility that the servicing technician willmis-diagnose the CPD source or waste man-power, resources, time and/ormoney performing additional testing chasing down the problem.

In addition, no additional hardware is needed to carry out the methods200, 250 disclosed herein—i.e., no additional hardware is required tomodify the test instrument's hardware. Likewise, the costs associatedwith a reverse test port and or additional drops are not needed either.In one or more embodiments, the methods 200, 250 may be ported topre-existing test instruments as part of a software upgrade. No boardspin or additional product cost would be required to implement thedisclosed principles. This means that the disclosed principles may bedeployed on tens of thousands of test instruments that are alreadydeployed in the field.

In one or more embodiments, testing may be performed at initial serviceinstallation, or upon the detection of service impairment.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example and notlimitation. It will be apparent to persons skilled in the relevantart(s) that various changes in form and detail can be made thereinwithout departing from the spirit and scope. In fact, after reading theabove description, it will be apparent to one skilled in the relevantart(s) how to implement alternative embodiments. For example, othersteps may be provided, or steps may be eliminated, from the describedflows, and other components may be added to, or removed from, thedescribed systems. Accordingly, other implementations are within thescope of the following claims.

In addition, it should be understood that any figures which highlightthe functionality and advantages are presented for example purposesonly. The disclosed methodology and system are each sufficientlyflexible and configurable such that they may be utilized in ways otherthan that shown.

Although the term “at least one” may often be used in the specification,claims and drawings, the terms “a”, “an”, “the”, “said”, etc. alsosignify “at least one” or “the at least one” in the specification,claims and drawings.

Finally, it is the applicant's intent that only claims that include theexpress language “means for” or “step for” be interpreted under 35U.S.C. 112(f). Claims that do not expressly include the phrase “meansfor” or “step for” are not to be interpreted under 35 U.S.C. 112(f).

What is claimed is:
 1. A computer-implemented method, said method beingperformed on a test instrument adapted to test for common pathdistortion at a downstream portion of a cable television network, saidmethod comprising: transmitting a first test signal having a firstfrequency into the downstream portion of the network; transmitting asecond test signal having a second frequency into the downstream portionof the network; measuring a first input signal at a third frequency;measuring a second input signal at a fourth frequency, the third andfourth frequencies being different frequencies and based on the firstand second frequencies; and determining that the downstream portion ofthe network is experiencing common path distortion based on acharacteristic of the first or second input signals.
 2. The method ofclaim 1, wherein the third frequency is a sum of the first and secondfrequencies and the fourth frequency is the difference between the firstand second frequencies.
 3. The method of claim 1, further comprisingdetermining a quiet zone and selecting the first and second frequenciessuch that the third and fourth frequencies fall within the quiet zone.4. The method of claim 3, wherein determining the quiet zone comprises:measuring ingress at the downstream portion of the network; determiningthat the measured ingress is above a predetermined threshold; andidentifying one or more frequencies where the measured ingress is flatcompared to other frequencies.
 5. The method of claim 3, whereindetermining the quiet zone comprises: measuring ingress at thedownstream portion of the network; determining that the measured ingressis above a predetermined threshold; and identifying one or morefrequencies where the measured ingress is a lowest compared to otherfrequencies.
 6. The method of claim 1, wherein the characteristic of thefirst or second input signals is an amplitude of the signals.
 7. Themethod of claim 6, wherein determining that the downstream portion ofthe network is experiencing common path distortion comprises determiningthat the first or second input signals have an amplitude exceeding apredetermined threshold.
 8. The method of claim 1, further comprisingoutputting an indication from the test instrument when it is determinedthat the downstream portion of the network is experiencing common pathdistortion.
 9. The method of claim 8, wherein the output indicationcomprises one or more of a visual indication, audio indication or hapticindication.
 10. A test instrument for testing for common path distortionat a downstream portion of a cable television network, said testinstrument comprising: a storage device; and a processor executingprogram instructions stored in the storage device and being configuredto: transmit a first test signal having a first frequency into thedownstream portion of the network; transmit a second test signal havinga second frequency into the downstream portion of the network; measure afirst input signal at a third frequency; measure a second input signalat a fourth frequency, the third and fourth frequencies being differentfrequencies and based on the first and second frequencies; and determinethat the downstream portion of the network is experiencing common pathdistortion based on a characteristic of the first or second inputsignals.
 11. The test instrument of claim 10, wherein the thirdfrequency is a sum of the first and second frequencies and the fourthfrequency is the difference between the first and second frequencies.12. The test instrument of claim 10, wherein the processor is furtheradapted to determine a quiet zone and select the first and secondfrequencies such that the third and fourth frequencies fall within thequiet zone.
 13. The test instrument of claim 12, wherein the processoris adapted to determine the quiet zone by: measuring ingress at thedownstream portion of the network; determining that the measured ingressis above a predetermined threshold; and identifying one or morefrequencies where the measured ingress is flat compared to otherfrequencies.
 14. The test instrument of claim 12, wherein the processoris adapted to determine the quiet zone by: measuring ingress at thedownstream portion of the network; determining that the measured ingressis above a predetermined threshold; and identifying one or morefrequencies where the measured ingress is a lowest compared to otherfrequencies.
 15. The test instrument of claim 10, wherein thecharacteristic of the first or second input signals is an amplitude ofthe signals.
 16. The test instrument of claim 15, wherein the processoris adapted to determine that the downstream portion of the network isexperiencing common path distortion by determining that the first orsecond input signals have an amplitude exceeding a predeterminedthreshold.
 17. The test instrument of claim 10, wherein the processor isfurther adapted to output an indication from the test instrument when itis determined that the downstream portion of the network is experiencingcommon path distortion.
 18. The test instrument of claim 17, wherein theoutput indication comprises one or more of a visual indication, audioindication or haptic indication.